In a conventional device for performing welding by automatically feeding a welding wire and alternately repeating short circuits and arcs, a short circuit is urged to open by supplying a large current to increase the short-circuit current with time until the short circuit opens and an arc occurs.
In the case, however, that the welding wire is made of a material not easily meltable due to its low intrinsic resistivity such as aluminum, the welding wire continues to be semi-molten even when a large current is supplied to open a short circuit. In this case, the short circuit does not open, thus failing to create an arc.
If the short circuit does not smoothly open, welding may be interrupted either due to the buckling of the welding wire or due to the stop of the welding device in response to an overcurrent protection function. The overcurrent protection function protects the welding device and welding jigs from overcurrent.
One well-known method for preventing wire buckling and hence avoiding welding from being interrupted is to monitor the motor current of the wire feeding motor and to stop the feeding of the welding wire (see, for example, Patent Literature 1).
The above-mentioned overcurrent protection function has been conventionally used to protect a welding device and welding jigs at an emergency, such as a short circuit between the tip of the welding wire and the welding output side.
FIG. 7 shows a schematic configuration of a conventional welding device. FIG. 8 shows temporal changes of a welding current and an AS signal which indicates an arc state and a short-circuit state in the conventional welding device. The conventionally used overcurrent protection function is described as follows with reference to FIGS. 7 and 8. FIG. 7 shows the overall structure of the conventional welding device having the overcurrent protection function, and FIG. 8 shows the relation between the waveform of a welding output and various timings in the conventional welding device.
The operation of the welding device shown in FIG. 7 is described with reference to FIG. 8. In the following description, the welding device is a consumable electrode arc welding device which performs welding by alternately repeating short circuits and arcs.
In welding device 1 shown in FIG. 7, welding output unit 2 provides a welding output. Output controller 3 controls welding output unit 2. Current detector 4 detects a welding current. Voltage detector 5 detects a welding voltage. Feeding motor 6 feeds welding wire 9. Feed roller 8 feeds welding wire 9. Base material 12 is an object to be welded, and welding torch 10 creates arc 11 between welding wire 9 and base material 12. Wire feed controller 13 controls feeding motor 6. AS determination unit 14 determines based on the output of voltage detector 5 whether it is an arc state where welding wire 9 and base material 12 are arced, or a short-circuit state where welding wire 9 and base material 12 are short circuited.
In FIG. 7, welding output unit 2 receives a commercial electric power from outside welding device 1, and outputs a welding voltage and a welding current by inverter operation in accordance with welding conditions.
Current detector 4, which can be formed of a current transformer (CT), detects a welding current. Voltage detector 5, which measures the voltage across the output terminals of welding device 1, detects a welding voltage.
AS determination unit 14, which may be composed of a CPU, receives a voltage detection signal from voltage detector 5. When the welding voltage reaches a predetermined detection level (for example, 15V) in a short circuit state, AS determination unit 14 determines that the AS signal indicates an arc state (high level). When, on the other hand, the welding voltage reaches a predetermined detection level (for example, 10V) in an arc state, AS determination unit 14 determines that the AS signal indicates a short-circuit state (low level).
Wire feed controller 13 controls the rotation of feeding motor 6. Feeding motor 6 is connected to feed roller 8 whose rotation allows welding wire 9 to be pressure-fed at a basic feed speed according to welding conditions.
Welding torch 10 supplies the output of welding output unit 2 to welding wire 9, thereby creating arc 11 for welding between the tip of welding wire 9 and base material 12.
In FIG. 8, time point E1 is when a short circuit occurred. Time point E2 is when an arc occurred after time point E1. Time point E3 is when a short circuit occurred after time point E2. Time point E7 is when the current of the short circuit that had occurred at time point E3 reached a predetermined overcurrent protection detection current value IOC. Time point E8 is when an overcurrent protection detection period TOC has passed since time point E7, and the overcurrent protection function comes into operation.
As shown in FIG. 8, since time point E3 at which the short circuit occurred, the short-circuit current is controlled to increase with an appropriate gradient in order to open the short circuit. Opening a short circuit requires outputting an extremely high welding current. The short-circuit current is clipped at a maximum output current value IMAX (for example, 550 A) as an upper limit which is determined according to the performance of welding device 1. The short-circuit current continues to have the maximum output current value IMAX until the short circuit opens and an arc occurs.
Assume that the welding current reached the overcurrent protection detection current value IOC (for example, 500 A) at time point E7, and that the overcurrent protection detection period TOC (for example, 300 msec) passed at time point E8. Also assume that the short circuit did not open between time points E7 and E8, and that the welding current continued to be equal to or more than the overcurrent protection detection current value IOC. In this case, welding device 1 forcefully terminates its output to protect itself and welding jigs from overcurrent.
Even when the short circuit cannot be opened by the time the overcurrent protection detection period TOC passes, welding wire 9 continues to be fed to base material 12. The feeding of welding wire 9 continues until the overcurrent protection function comes into operation to stop the output of welding device 1 and the feeding of welding wire 9. This causes welding wire 9 to be pushed deeply into the weld pool of base material 12, making it much more difficult to open the short circuit.
Failing to open the short circuit during the predetermined period causes the overcurrent protection function to come into operation, and forcefully terminates welding device 1. The forceful termination of welding device 1 interrupts the welding. This often damages base material 12, causes quality issues such as welding defects, or reduces production efficiency.